18Watt.com

Hi everybody,
I’ve built a few F-type amps over the years and have finally come round to building an “18-watt 36-watter”. Sounds really good, perhaps a bit trebly even with Greenbacks or Lead 80s (though SC guitars), but with the treble pot 90% down it’s ok.

What’s a bit weird is the fact that the bass pot is almost completely ineffective. The mid pot is wildly effective in the low mids, including added gain as it’s turned up, but nothing happens at the bottom end when the bass pot is cranked. Is this perhaps typical of this tone stack?

I’m aware that the bass pot is wired as a variable resistor, with one side not connected to anything, according to the JMP schematic as posted in the D/L section of this site. Is this the correct wiring for the intended tone stack? Or do I have a faulty (new) pot? I’m 100% certain that my wiring follows the schematic down to a T.

Anyway, thanks for this really helpful site and your help.

Recky

Hi,

That's definitely not normal behavior. Based on the photos, I believe the problem is most likely due to soldering issues. Please read the soldering thread in my signature for best practices and examples.

I highlighted some issues on your build:
You should be more careful with component leads. Careful they're not too long, not touching ANYTHING they shouldn't, etc...

Your output jack should be isolated from the chassis (ideally all jacks should be isolated).

Most of the arrows are examples of soldering that should be improved. Hopefully the soldering thread will illustrate this better.

Thanks,
Josh

Hi Josh,

I never expected to get a reply that quickly. Thanks very much indeed!

While admittedly most of the arrows point to areas that do look a bit dodgy in the picture, they're actually fine. The input cap rests high above the resistor, the lone turret with nothing attached to it is an unused turret that I couldn't be bothered to remove, the blob of solder on one of the transformer leads is just that, a blob of solder that scrapes right off, and the insulation is perfectly intact. All solder joints are rock solid. I do admit, things could be quite a bit neater overall. I have read all your information behind the links you mention. Great advice which I will follow as best as I can!

This build has had no issues from the second it was powered up except the strangely inefficient bass control. Provided everything else is connected as required per your schematic, would you have any idea just what may render the pot ineffective? I have looked at all kinds of tone stacks, and your typical F-type TMB usually has pin 1 of the bass pot connected to the intersection of the .022uF bass cap and pin 3 of the treble pot. (I'm going off memory here...) At the same time, a variable resistor need not be connected on both sides. This is a bit confusing to me.

Since I usually measure each resistor before soldering it into the circuit, I skip checking resistors' values in a highlighter check-up and just check the caps. So I'm going to check that 56K resistor again. Maybe I messed the tone stack up right there.

Again, thanks for your valuable help and a happy new year from Germany,
Recky

Ah, one thing I forgot:

You say the output jack should be isolated from the chassis. I've never seen that done on any 7ender amps. I presume the black plastic Marshall-type jack sockets are isolated by default. How would you isolate these open Switchcraft sockets? Plastic washers? The input sockets are the same, so should these be isolated from the chassis, too? I'm guessing this is to avoid ground loops..??

Thanks very much again,
Recky

Yes, the input jacks also need to be isolated from the chassis, to avoid ground loops. The ground connections on the input jacks all need to be separately-connected to a very close-by ground lug, which is also where the preamp ground bus terminates. See Josh's information on modern grounding.

Regarding the bass control, I've seen posts pointing out that the typical Marshall tone stack is an evolution of the F***er 5F6A Tweed Bassman's tone stack, since that's the amp, the Marshall JTM45 was derived from. No big surprise, the classic Marshall tone stack works best in the bass guitar frequency range!

So, I tweaked the TMB tone stack values, with the help of Duncan's Tone Stack Calculator, to make it work better in the six-string range, as well as to reduce its insertion loss and make it work well fed either from a cathode follower or triode gain stage-anode output and called it the "Jalapeno" tone stack. If you see file attachment references to the SuperLite TMB, please ignore them. They got attached by mistake and have the wrong tone stack values The one here in the post is correct

BTW, due to my day job, I've traveled extensively in Germany and love the place

Hey Phil,
Thanks for your reply! I’m definitely going to print that tone stack of yours for future reference.
I “fixed” that ineffective bass pot. Due to gross stupidity on my part I missed the fact that I had installed a .1nF instead of the .01nF cap in two highlighter checks…
Now the amp sounds glorious!!!

I live in the Eifel range, not far from Cologne. My restaurant was destroyed in the flooding, but I’m going to rebuild. While I’m waiting for things to happen I’ve gone back to building amps. If ever you are in the area, pop in for a beer!

Cheers,
Recky

recky wrote: ↑

Mon 01/10/22 3:16 pm

I missed the fact that I had installed a .1nF instead of the .01nF cap in two highlighter checks…
Now the amp sounds glorious!!!

And that's what we expect here! Congratulations!

I hope you're going to post some sound clips or a demo video.
And I hope the rebuild and reboot of your restaurant goes well
My previous employer had an office in Düsseldorf - not too far from you But now I'm in Canada.